Conflict of interest: C. Thamrin has a patent WO 2006130922 A1 issued, which is broadly relevant to the work; and has intellectual property arrangements with Thorasys Medical Systems and Restech srl relating to research collaborations, but does not have any financial relationships with either company. Conflict of interest: R.L. Dellacà reports grants from and is a shareholder in Restech, personal fees for lectures from Philips heathcare, outside the submitted work; has a patent on the detection of EFL by FOT, with royalties paid to Philips Respironics and Restech srl, a patent on monitoring lung volume recruitment by FOT, with royalties paid to Vyaire, and a patent on early detection of exacerbations by home monitoring of FOT, with royalties paid to Restech; and is co-founder and shareholder of Restech srl, a spin-off company of the Politecnico di Milano University producing medical devices for lung function testing based on FOT. Conflict of interest: G.L. Hall has nothing to disclose. Conflict of interest: D.W. Kaczka reports grants from US Department of Defense (W81XWH-16-1-0434), National Institutes of Health (R41 HL140640) and ZOLL Medical Corporation, salary support from University of Iowa, and is shareholder and member of a scientific advisory board for Monitor Mask, Inc., outside the submitted work. Conflict of interest: G.N. Maksym reports non-financial support from Thorasys prior to the submitted work; and is contributing inventor on the following patents 1) respiratory device (design patent), 2) respiratory device filter, 3) method of assessment of airway variability in airway hyperresponsiveness, 4) piezoelectric beam bending actuated device for measuring respiratory system impedance, each of these issued to Thorasys. Conflict of interest: E. Oostveen has nothing to disclose. Conflict of interest: S.J. Simpson has nothing to disclose. Conflict of interest: G.G. King reports consultancy fees for talks, chairmanship, advisory boards and conference sponsorship from AstraZeneca, Boehringer Ingelheim, Chiesi, GlaxoSmithKline, Menarini, MundiPharma and Novartis; and unrestricted research grants from NHMRC, Boehringer Ingelheim, CycloPharma, GlaxoSmithKline, Menarini, MundiPharma, and philanthropic individuals and societies; and has a collaborative research agreement with Restech, Italy.